1. Field Of The Invention
The present invention relates in general to the doping of semiconductor materials, and more particularly, to the use of triisopropylindium as a dopant precursor in the chemical vapor deposition of II/VI semiconductor materials.
2. Related Art
The II/VI semiconductor materials such as mercury cadmium telluride (HgCdTe) and cadmium telluride (CdTe) have many applications both in the military as well as the commercial sector. A particularly important application of the II/VI semiconductor materials is in infrared detectors. There exist a variety of binary semiconductor systems such as HgTe and CdTe as well as many ternary and quaternary semiconductor systems that have been investigated for applications in infrared emitters and detectors operating in the 3-5 .mu.m and 8-12 .mu.m spectral ranges. These wavelength ranges are particularly important because they are natural windows in the atmosphere for infrared transmission. In addition, many II/VI semiconductor materials are also potential candidates for efficient solar cells and other advanced opto-electronic devices.
Many of these advanced applications require controlled extrinsic doping of both p-type and n-type semiconductor materials. The related art has demonstrated metal-organic chemical vapor deposition (MOCVD) of p-type HgCdTe doped with arsenic using both arsine and tertiarybutylarsine. However, low level doping of n-type HgCdTe in the range of 10.sup.14 atoms per cm.sup.3 to 10.sup.15 atoms per cm.sup.3, remains more of a problem.
A variety of approaches have been used to control the doping of n-type HgCdTe grown by metal-organic chemical vapor deposition (MOCVD). These include group III doping onto the group II site using Al, Ga and In and group VII doping onto the group VI site using iodine. A common problem with the precursors developed for III-V MOCVD is that their saturated vapor pressures are too high to be handled conveniently as dopant precursors. Lower dopant concentrations can be obtained from an effuser source but there is less flexibility in the range of concentrations from vapor pressure simply controlled by the source temperature. A potentially more serious problem is unwanted reaction processes that cause a memory effect where doping will persist for a number of growth runs following its introduction.
Indium is the dopant of choice for producing n-type HgCdTe due to its slow diffusion in HgCdTe compared to other n-type dopants. It has been used to produce n-type HgCdTe grown by MOCVD using both an interdiffused multilayer process and directed alloy growth. Most of the work to date has utilized trimethylindium as the indium dopant precursor.
However, as disclosed in U.S. patent application Ser. No. 08/027,314, Filed Feb. 25, 1993, now U.S. Pat. No. 5,346,852, there are several problems generally associated with the use of trimethylindium. Since trimethylindium is a solid at or below room temperature, the effective vapor pressure of trimethylindium in a conventional bubbler changes with time due to changes in the surface area of solid. This often results in transport problems. Furthermore, the use of trimethylindium in low temperature processes can result in unintentional carbon impurity incorporation, due to the formation of methyl radicals during pyrolysis, which can be deleterious to the ultimate performance of a semiconductor device.
To that end, trimethylindium is not a suitable precursor for doping HgCdTe at low concentrations. Because of its relatively high vapor pressure the lowest doping achieved to date with trimethylindium is in the mid 10.sup.16 cm.sup.-3. In addition, trimethylindium has shown a significant `memory` effect, where indium has been detected in un-doped growths.
Even though trimethylindium has been used successfully as a source compound for indium containing semiconductor materials, trimethylindium has several noted problems when used as a low level n-type dopant in HgCdTe. Thus a given compound which may demonstrate acceptable results when used as a source compound for semiconductor materials does not mean that the same compound can be used as dopant source for semiconductor materials with equally good results.
What is desired is a liquid dopant precursor which can provide a constant quantity of indium and has a very low vapor pressure at reasonable temperatures to allow doping at low carrier concentrations.